A PTC device is widely used as a protection device to protect an electrical element constituting an electrical apparatus, for example a secondary battery cell, or a circuit constituting an electrical apparatus, when an excessive current flows through various electrical apparatuses. The PTC device is also widely used to interrupt an electrical current flowing through the electrical apparatus when the electrical element fails, and as a result, a temperature of the electrical apparatus rises to an abnormally high temperature.
An example of such a PTC device is shown in FIG. 3 in a schematic cross-sectional view. The PTC device 110 comprises a PTC component 112 and leads 114 and 116 respectively disposed on both sides thereof. The illustrated PTC device 110 is disposed, for example, in a charge/discharge circuit of a secondary battery cell. The PTC device is used as a circuit protection device to interrupt an excessive current flow when an excessive current flows through the circuit. In the illustrated embodiment, the lead 114 is electrically connected to a substrate having the protection circuit, and the lead 116 is electrically connected to the secondary battery cell.
The PTC component 112 comprises a PTC element 118 made of an electrically conductive PTC composition and metal electrodes 120 and 122 respectively disposed on both sides thereof, and these are typically formed to be integral by thermal compression.
The metal electrode 120 of the PTC component 112 is electrically connected to the substrate-side lead 114 with a solder paste 124 disposed therebetween. The metal electrode 122 is electrically connected to the cell-side lead 116 with a solder paste 124 disposed therebetween.
To produce the PTC device, for example, the solder paste 124 is disposed on the substrate-side lead 114, then the PTC component 112 is disposed such that the metal electrode 120 is positioned the solder paste 124. Then, the solder paste 124 is disposed on the metal electrode 122 and the cell-side lead 116 is disposed thereon to form an assembly. Then, the assembly is passed through a reflow furnace to melt the solder paste, and then the molten solder paste is cooled to be solidified to obtain the PTC device.
After obtaining the PTC device by passing the assembly through the reflow furnace to integrally connect with the solder paste, an oxygen barrier layer 126 is formed on a side portion of the PTC component. In order to do this, an epoxy resin is applied to the side portion of the PTC device while holding the obtained device by another jig to form a coating layer, and the coating layer is cured (or hardened) to form the oxygen barrier layer 126.
International Patent Publication No. WO2007/052790 is a prior patent reference.